The present invention relates to a measuring platform consisting of components, in particular a cover plate and a base part, joined by at least one force transducer between them.
Such measuring platforms, as are employed typically in biomechanics, serve to measure forces in one or more directions, and occasionally torques as well. For example the foot forces of a person walking are measured, whereby the vertical component, the horizontal component in the direction of walking, and the lateral force component are analyzed. The cover plate is usually level with the ground, while the base part may consist of a base plate or frame and be installed permanently in the floor, in a recess in a gangway for example. However the base part may also consist of only four feet for instance.
A fully formed measuring platform enables the three components F.sub.x, F.sub.y and F.sub.z of a force F acting on the platform L to be measured, also the three components M.sub.x, M.sub.y and M.sub.z of the resulting moment vector. Usually the measuring platform has four force transducers arranged in a rectangle, with each transducer capable of measuring the three components F.sub.x, F.sub.y and F.sub.z of a force acting on it from any direction. The x and y coordinates run parallel with the sides of the rectangle formed by the four force transducers, while the z coordinate stands vertical to them. The coordinate origin is generally located in the medium plane of the transducers. The z axis of the coordinate system is located so that it pierces the surface of the cover plate in the middle of the rectangle formed by the four force transducers. The electrical signals produced by the four force measuring cells are generally added partially (in parallel circuitry) and evaluated in a calculating unit so that not only the required force components, but also the components a.sub.x and a.sub.y of the force application point and the torque T about the z axis, can be determined. The formulas for measuring and calculating the forces, force application points and torques are known, for example J. Tichy and G. Gautschi, Piezoelektrische Messtechnik, Springer Verlag, pp. 175-181. Another example is Swiss Patent PS 502 590, which in addition gives an example of the circuitries to be employed.
The known measuring platforms generally comprise four piezoelectric force transducers, though platforms with more or fewer force transducers are possible also. Thus for example, a platform with three force transducers is used for measuring metal cutting operations. Instead of piezoelectric transducers, force transducers based on other principles are possible also, such as resistance, capacitance etc. It should be noted that these alternatives may be employed for the present invention also.
When a pressure acts on a conventional platform, it bends downward. The pressure does not have to be a single force, it can just as well be the resultant of several individual forces. Moreover, it will not generally be a pure pressure force but the vertical component acting in the z direction of a force at a polyhedral angle to the platform. The bending of the measuring platform has metrological consequences, as shown in FIG. 1. In this drawing, 1 denotes the cover plate of the measuring platform, 2 the force transducers (only two are shown), and 3 the base plate A force F.sub.z acting vertically to the platform bends this down. The amount of this deflection depends on the rigidity of the cover plate, which in turn depends on the dimensions of this plate and the material used for it. In any case, the pressure force F.sub.z causes bending moments and shear forces designated My.sub.1, My.sub.2, Fx.sub.1 and Fx.sub.2 in FIG. 1, whereby the indices refer to the individual force transducers. Owing to the deflection, the loading of the transducers is unequal, as is shown schematically in FIG. 2. Their sensitivity, linearity and stability are impaired by this.
Significant also are the adverse metrological effects of the torques My.sub.1 and My.sub.2 and the shear forces Fx.sub.1 and Fx.sub.2. Numerous applications for such measuring platforms, in biomechanical examinations for example, call for the definition as exactly as possible of the coordinates of the force application point a.sub.x and a.sub.y (not shown), measured on the surface of the cover plate 1 (FIG. 1). Calculations reveals that the measuring error .DELTA.a.sub.x,y for a.sub.x and a.sub.y is proportional to the difference of the torques .DELTA.M.sub.x,y about the corresponding axis (x or y). With any eccentric force application point of F.sub.z with the coordinates a.sub.x,y, the measuring error .DELTA.a.sub.w,y may be considerable, so that it cannot be ignored.
The shear forces Fx.sub.1 and Fx.sub.2 shown in FIG. 1 generally cause "cross-talk" onto the axial force Fz, which is due to the imperfection of the transducers. If a cross-talk of typically 2% is assumed from the shear force onto the axial force for the transducers, in the output signal of each transducer 2% of Fx.sub.1 and of Fx.sub.2 will be added to the measurements of Fz.sub.1 and Fz.sub.2 respectively. The resulting measuring error may be considerable, apart from the fact that Fx.sub.1 and Fx.sub.2 are not linear functions of Fz. There may also be cross-talk from Fx.sub.1 include and F.sub.2 onto the force components Fy.sub.1 and Fy.sub.2, which are then measured error.
Therefore, it is an object of the invention to provide a measuring platform, on which the occurrence of measuring errors due to the bending of the cover plate of the platform under loading is avoided wholly or at least to a very large extent.
This objective is achieved by connecting the force transducer or transducers to one of the cover plate or the base part or base plate essentially rigidly, and connecting the transducer with the other part movably, so that bending moments and shear forces resulting from the deflection of the cover plate can be compensated for substantially, preventing them causing any of the measuring errors named. For example the two expressions .DELTA.a.sub.x,y and .DELTA.M.sub.x,y become equal to nil, because the moments become zero.
According to developments of the invention, the connection essentially free of fixing moments between force transducer and the relevant part of the measuring platform comprises a suitable joint device, such as self-aligning ball bearing, radial ball-and-socket joint, axial ball-and-socket joint etc. or alternatively a suitable elastic supporting element, such as a hydraulic element or intermediate link, or a vessel filled with liquid. According to another development of the invention, the connection may also take the form of a flexible supporting tube fixed to one of the components. The force transducer is , preferably braced against one of the components by a clamp screw that may be flexible.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.